Sewage sludge acidogenic fermentation for volatile fatty acids production: long-term monitoring and ADM1-based modelling of a full-scale plant

This study explores the feasibility of producing volatile fatty acids (VFA) from sewage sludge via acidogenic fermentation, validating the process at full-scale with a fermentation reactor of 315 m3. The study was conducted at an Italian wastewater treatment plant (WWTP) serving 100,000 P.E. The fermentation process was monitored across six distinct operational periods over a four-year time span. The results revealed that high Organic Loading Rates (OLR), up to 14.0 kg VS m³ d⁻¹, and low Hydraulic Retention Times (HRT), below 5 days, significantly enhanced VFA production. Periods characterized by high temperatures (up to 30°C) and optimal OLRs achieved acidification yields of 0.32 g VFA-COD g⁻¹ VS. Considering the complexity and heterogeneity of the waste sludge and the diversity of microbial communities involved, the process could benefit from the use of mathematical models to gain a deeper understanding and to optimize the process performance. The IWA-Anaerobic Digestion Model 1 (ADM1) was used to simulate the full-scale acidogenic fermentation system and fit experimental data. A parameter identification procedure was used to select the most sensitive parameters for calibration, highlighting the crucial role played by the stoichiometry of amino acids and sugar conversion into VFAs, as well as by the hydrolysis kinetics. The calibrated model significantly improved the prediction of soluble chemical oxygen demand (sCOD), propionate butyrate, and valerate concentrations.